Condenser for low boiling fractions in rectifying or distilling columns

ABSTRACT

A condenser for the head fraction in distilling or rectifying columns has vertical risers which are connected to and extend upwardly from the shell of the column. The risers support pairwise arranged groups of horizontal, vertical, or sloping heat-exchanging pipes which extent outwardly from and receive the fraction from the respective risers. Each pair of groups carries A housing which serves to shield the pipes from the wind and precipitation and supports an adjustable axial fan which circulates atmospheric air across the pipes to effect condensation of the fraction. The compartments between the heighboring pairs of groups are partially enclosed to prevent immediate recirculation of heated air. The condensate is collected in chambers which return some of the condensate into the shell by gravity flow.

L 1972 H KASSAT ETAL 3,703,592

CONDENSER FOR LOW BOILING FRACTIONS IN REOTIFYING OR DISTILLING COLUMNS Filed July 27. 1970 7 Sheets-Sheet 1 Fig.1

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CONDENSER FOR LQW BOILING FRACTIONS IN File Ju y 7 RECTIFYING OR DISTILLING COLUMNS 7 Sheets sheet 2 Fig.2

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CONDENSER FOR LOW BOILING FRACTIONS IN RECTIFYING OR DISTILLING COLUMNS Filed July 27. 1970 '7 Sheets-Sheet 4 Fig.4

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CONDENSER FOR LOW BOILING FRACTIONS IN Filed y 27' 1970 RECTIFYING OR DISTILLINO COLUMNS 7 sheets sheet a Fig. 5

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Umted States Patent US. Cl. 202-185 B 24 Claims ABSTRACT OF THE DISCLOSURE A condenser for the head fraction in distilling or rectifying columns has vertical risers which are connected to and extend upwardly from the shell of the column. The risers support pairwise arranged groups of horizontal, vertical or sloping heat-exchanging pipes which extend outwardly from and receive the fraction from the respective risers. Each pair of groups carries a housing which serves to shield the pipes from wind and precipitation and supports an adjustable axial fan which circulates atmospheric air across the pipes to effect condensation of the fraction. The compartments between the neighboring pairs of groups are partially enclosed to prevent immediate recirculation of heated air. The condensate is collected in chambers which return some of the condensate into the shell by gravity flow.

BACKGROUND OF THE INVENTION The present invention relates to distilling or rectifying columns in general, and more particularly to improvements in outdoor columns of the type wherein the upper end portion of the upright shell of the column supports a condenser for the low boiling or head fraction of the processed material. Still more particularly, the invention relates to improvements in air-cooled condensers for use in or on distilling or rectifying columns.

Air-cooled condensers which are carried by the shells of rectifying or distilling columns have found widespread acceptance because they occupy space which is readily available, namely, on top of the shell. Thus, the shell can serve as a convenient carrier for the condenser so that the latter need not be provided with its own supporting frame. Furthermore, the low boiling or head fraction (particularly vapors) must cover a relatively short distance on its way from the shell into the pipes of the condenser so that the losses in pressure are very low. Still further, the condensate can be returned into the shell by gravity flows so that the admission of reflux can be effected without resorting to pumps or the like.

The trend in the chemical industry, as well as in nearly all other industries, is to resort to ever larger machines and apparatus .in order to reduce the cost and to raise the output. Such considerations have led to the construction of giant outdoor rectifying and distilling columns which must be equipped with correspondingly enlarged condensers for the head fraction. This creates serious problems because the capacity of a column can be more readily enlarged than the capacity of presently known condensers which are mounted on top of the shell. As a rule, a conventional condenser comprises batches of heatexchanging pipes which form a polygon having its center on the axis of the shell. It is also known to place the pipes around the upper end portion of the shell. If the diameter of the shell is increased by 100 percent, the capacity of the column is increased four-fold but the capacity of the heat-exchanging piping (in polygonal dis- Patented Nov. 21, 1972 ice tribution) increases only twofold. It is to be kept in mind that the dimensions of the condenser cannot be increased at will since the condenser must be mounted on and safely supported by the upper end portion of the shell. This imposes limits on the extent to which the parts of the condenser can project radially beyond the outline of the shell. If the dimensions of the condenser are increased beyond the capacity of the shell to support the thus enlarged condenser, the column must be provided with auxiliary frames to carry the weight and to prevent excessive stressing of the shell. The initial and maintenance cost of auxiliary frames as well as their space requirements render such condensers unsuitable for many purposes.

Another drawback of conventional condensers wherein the heat-exchanging pipes form a polygon is that their external surfaces exchange heat with cool atmospheric air in the area which directly surrounds the condenser. Thus, the operation of the condenser is influenced by the atmospheric conditions which can lead to highly unsatisfactory results. The condensation of vapors in various sections of the condenser progresses at a different rate. especially when one of its sides is exposed to strong winds or driving snow or rain. The wind, rain or snow can cause more rapid condensation at one side of the condenser and this can lead to undercooling and in some instances to freezing of condensate. Even if the rectifying or distilling plant is located in a milder climate where the atmospheric conditions are reasonably stable, the influence of surrounding air can suffice to prevent uniform condensation in each zone of the condenser. Therefore, such conventional condensers cannot be used for controlled cooling of many types of head fractions, namely, such fractions which are to yield condensate whose temperature must be maintained within a narrow range. No effective method and apparatus for accurate control of the temperature of condensate in large outdoor air-cooled condensers are believed to be available at this time.

SUMMARY OF THE INVENTION An object of the invention is to provide a distilling or rectifying column with a novel and improved condenser whose output, reliability and capacity are less dependent on its transverse dimensions and which is capable of elfectively cooling head fractions in large, medium-sized or small columns.

Another object of the invention is to provide a rectifying or distilling column wherein the capacity of the condenser can be increased at the same rate as the capacity of the shell to furnish a head fraction but without necessitating undue enlargement of the condenser radially of the shell.

A further object of the invention is to provide a condenser whose condensing action is less dependent on or fully independent from the changes in atmospheric conditions.

An additional object of the invention is to provide a condenser which can be used on the shells of existing columns as 'a superior substitute for presently known condensers.

Still another object of the invention is to provide a novel and improved adjusting system for regulating the cooling action in air-cooled condensers for use with or on distilling or rectifying columns.

A further object of the invention is to provide a condenser 'wherein the heat-exchanging piping is distributed and connected to the sources of head fraction and to the condensate-collecting means in a novel and improved Way.

An ancillary object of the invention is to provide a condenser wherein winds, rain, snow, hail and other atmospheric phenomena are prevented from influencing the progress of condensation.

Still another object of the invention is to provide a condenser whose design constitutes a radical and advantageous departure from the design of presently known condensers for use on the shells of rectifying or distilling columns.

Another object of the invention is to provide a condenser whose components can be used for the assembly of larger or smaller condensers, depending on the output of the associated column, and wherein the condensation can be individually controlled in each of several condenser sections.

The invention is embodied in a distilling or rectifying column which comprises an upright shell wherein the hot lower boiling or head fraction rises toward and into the upper end portion of the shell, a plurality of tubular risers which extend upwardly from and communicate with the upper end portion of the shell to receive the head fraction, and a novel condenser comprising a plurality of discrete condenser units, one for each riser and each including several groups of heat-exchanging pipes having uneven external surfaces. The pipes extend outwardly from and communicate with the respective risers, and the condenser further comprises means for circulating a fluid coolant along the external surfaces of the pipes to withdraw heat from and to thus effect condensation of the head fraction which enters the pipes by way of the respective risers, and collecting means for receiving condensate from the pipes. Such collecting means can return a portion of the condensate by gravity flow as reflux into the shell of the rectifying or distilling column.

Each unit of the condenser preferably comprises two groups of pipes and such groups preferably form the sides of an equilateral or isosceles triangle whose apex is connected with the respective riser. Such distribution of the pipes renders it possible to accommodate a substantial number of groups in a small area so that the condenser need not extend well beyond the outline of the shell.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved condenser itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a plan view of a rectifying or distilling column comprising a condenser which embodies one form of the invention, certain parts of the condenser being broken away;

FIG. 2 is a partly elevational and partly vertical sectional view as seen in the direction of arrows from the line IIII of FIG. 1;

FIG. 3 is an enlarged fragmentary perspective view of the structure shown in FIGS. 1 and 2, with parts of the condenser broken away;

FIG. 4 is a plan view of a second column which comprises a modified condenser, parts of the condenser being broken away; I

FIG. 5 is a partly elevational and partly vertical sectional view as seen in the direction of arrows from the line VV of FIG. 4;

FIG. 6 is an enlarged fragmentary perspective view of the structure shown in FIGS. 4 and 5; and

FIG. 7 is a diagrammatic view of a condensing unit and of the means for adjusting the cooling action in dependency on the temperature of the condensate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIGS. 1 to 3, there is shown a distilling or rectifying column having an upright shell 3 whose upper end portion includes a cap 13 and supports'a condenser 1 embodying one form of the present invention. The condenser comprises several condensing sections or units 2 each of which is supported by one of six equidistant upright tubular risers 4 which are welded to the upper end portion of the shell 3 and extend upwardly beyond the cap 13. Thus, the shell 3 supports all component parts of the condenser 1. The axes of the risers 4 intersect a circle which is concentric with the circular outline of the shell 3. The risers 4 have the same length and diameters and they extend very little beyond the outline of the shell 3.

Each condensing unit 2 comprises two mirror symmetrical groups 5 of heat-exchanging pipes 14, and the groups 5 of each unit 2 form the sides of an equilateral or isosceles triangle whose apex is located on or close to the axis of the respective riser 4. It will be seen that at least the major portions of the groups 5 extend outwardly beyond the outline of the shell 3. The heat-exchanging pipes 14 have uneven (preferably finned or ribbed) external surfaces to enlarge the area of contact with the surrounding air, and such pipes are assembled into bunches or subgroups 15 wherein the pipes 14 extend radically outwardly from the respective riser 4. Each group 5 has a row of vertically aligned distributing chambers 6 which receive the head fraction from the adjacent riser 4 and admit such fraction into the pipes 14 of the respective batch 15. Thus, one distributing chamber 6 is provided for each batch 15. The chambers 6 are connected with the respective riser 4 by horizontal conduits having portions 7 connected with the chambers 6 and portions 8 which are rigid with the riser 4. The portions 7, 8 have abutting flanges which are sealingly secured to each other by bolts or the like. The conduits 7, 8 constitute hollow connectors which support the weight of the respective groups 5.

The means for collecting the condensate which issues from the pipes 14 comprises collecting chambers 9 which are provided at the outermost ends of the groups 5, one for each of the batches 15, and discharge the condensate into vertical conduits 22. The conduits 22 admit condensate into horizontal conduits 10 which extend tangentially of the shell 3, and the conduits 10 admit condensate into radially extending horizontal conduits 11. The conduits 11 discharge the condensate into a ring-shaped conduit 12 which can admit some condensate by gravity flow into the shell 3 and the remainder to a collecting vessel, not shown.

Each condensing unit 2 is practically completely enclosed by a housing 16 preferably consisting of sheet metal. Each housing 16 leaves uncovered only those sides of the groups 5 (namely, of the pipes 14) which face toward the groups 5 of the adjoining units 2. The units 2 are separated from each other by compartments which receive air that passes along the external surfaces of the pipes 14 to effect condensation of the head fraction therein. As best shown in FIG. 3, each housing 16 comprises a triangular top wall 16a which extends between the respective riser 4 and the outermost ends of the groups 5, a vertical or nearly vertical outer wall 17 which is provided with a large centrally located inlet receiving a shielding ring 18 for an axial fan 19 or an analogous air circulating device, and a bottom wall 16a which is parallel with and similar to the top wall 16a. When the fan 19 is started, it draws air through the inlet within the shielding ring 18 and forces such air against those sides of the pipes 14 in the associated pair of groups 5 which face each other. The air flows across the uneven external surfaces of the pipes 14 and enters the adjoining compartments 75. It can be said that the clearances between the pipes 14 of the batches 15 constitute outlets through which the heated atmospheric air can enter the compartments 75 to be returned into the atmosphere.

In order to prevent uncontrolled mixing of heated air with cooler air which is drawn through the inlets of the outer walls 17, the condenser 1 further comprises enclosures or envelopes which partially enclose the compartments 75 to the extent necessary to prevent the recirculation of heated air. Such recirculation could unduly atfect the output of the --c'ondenser. Each envelope or enclosurecomprises a horizontal bottom panel 21 which extends between the lowermost portions of two adjoining groups and a vertical outergipanel 20,: which extends between the adjoining outer walls 17. Thus, hot air which has passed through the outlets between theheat-exchanging" pipes 14 and enters the compartments 75'must chow upwardly to be thus effectively separated'froni cool air which is drawn by the axial fans 19. The edges where'the outer panels 20ineet the outer walls 17 are indieated at 23.The numerals 24 denote the"edges -where' the bottom panels '21-;are-s ealingly secure'dto the lowermost portions of'theadjdining"groups 5: The outer'fpanels 2'0 of the enclosures for thev compartments 75 further serve'asa means 'fof preventin'g the wind, rain or'snow'from influencing the cooling of head fraction in the pipes 14'.

The reinforcing means for the condenser 1 comprises an upright reinforcing tube 26 which extends upwardly'from the c'ap"13-of the upperendportio'n and is coaxial with the shell 3, and radially outwardly extending connecting conduits 25 which convey the head fraction from the tube 26 into the risers 4.. Theconduits ,25. are so designed that they can take up tensional and preferably also'compressivel stresses. In order to further reinforce the condenser 1, the risers 4 can be connected-to each other by additional reinforcing conduits 25a (one shown in FIG. 1) which can take up tensional and preferably also compressivestresses and further serve to convey the head fraction.

The fans 19 force the atmospheric air to flow in directions indicated by the arrows at whereby such air cools the head fraction in the pipes 14 and enters the compartments 7 5, to leave by flowingin the directions indicated by arrows y; Since a the heated 'air, exhibits the tendency, to rise (throu lithe open tops ofthe enclosures for the compartments 75 it is highly unlikely to mix with cooler atmosplieric' v found that the panels 20', 21"of henclosures for the com-' mixing'of hot and cool air.

FIGS; 2 and 3 show the batches of, horizontal pipes 14.1" The batches 14 are preferably identical and each thereof is readily detachable from the remainde'r of the respective condensing unit 2.

Referring again to FIG. 1, the lower end portions of the risers 4 extend slightly beyond theoutline of the cylindrical wall 27 .of the shell 3.,Such lowerend portions of the risers 4 resemble pans and their concave open sides taregwelded or otherwise sealingly secured to the upper end portion of the shell. The-wall 27 is formed with apertures or perforations (shown at 35) which convey the hot head fraction from the "-interior of the shell into the risers 4. The shell 3 accommodates exchangeable trays 28. t 1

If desired, the lower end portions of the risers 4 can be provided with flanges which are bolted or otherwise secured to the cylindrical wall 27 of the shell 3. Also, the lower end portions of the risers 4 can be partially surrounded by the" wall 27, i.e., the circular outline of the cap '13 can intersect the' cross sections of the risers." 1he mounting 'of the risers'4 directly on the wall 27 of the shell 3' is preferred at this time because this'relieves the cap' 'l3 which merely supports the centrally located reinforcing tube 26. The wall 27 is capable ofcarrying 'substantial loads.

The 19 can be mounted in the top and/or bottom walls 'l'fia, 16b of the housings '16. Theenclosures (panels 20, 21) I are thend'esigned in such a way that they prevent mixing of hot air which-issues from the' compartment's 75 with 'coohair' which is being drawn into the spaces between the groups 5 of the condensingunits 2. I

"Since'the conduits 25 admit hot vapors into 'theupper ends of the risers-4, thethus adrriitted vapors preventthe" development" of a low-pressure' zorie' which tentls -to developif the risers receive" vapors exclusivety at rheir lower ends. Consequently, each of the superimposed bat'Cli'e's iS which is being drawn by thefans 19. It has been partme nts 75 are'highly effective to prevent undesirable of pipes 14 receives vapors at the same pressure and temperature. This brings about the additional advantage that the risers, pipes and conduits are uniformly heated to thus prevent the development of localized thermal stresses, especially at the points of connection.

The pipes 14 can be installed in such a way that they slope slightly outwardly and downwardly from the respective risers4. This facilitates the flow of condensate into thecollecting chambers 9. Such sloping pipes are often preferred to vertical pipes because the outflowing condensatecannot coat the entire internal surface of the pipe. The film of condensate on the internal surface reduces the efliciency of heat transfer between the head fraction and the' air around the pipe. The film of condensate on the internal surface of a sloping pipe merely forms a trough or channel and leaves the remaining portion of theinternal surface free for effective transfer of heat. Also, the sloping or horizontal heat-exchanging pipes can be vmade longer than vertical pipes to further increase the output of the batches by insuring a higher thermal efiiciency.

The length of the risers 4 depends on the number of batches 15 in the respective groups 5 and on the number and dimensions of pipes 14 in the individual batches. The horizontal conduits 7, 8 render it possible to securely connect each riser with a desired substantial number of batches-15 so-that such batches can contain long horizon-- tal or sloping pipes 14 with the resulting increase in output and efliciency of the condenser.

It is further Within the purview of the invention to replace the fans 19 with air circulating devices which suck air into the compartments 75 whereby such air flows across the pipes 15 of the adjacent groups 5 and is evacuated from the housings 16.

\ The basic design of the distilling or rectifying column of FIGS. 4 to 6 is very similar to that of the just described column. The main difference is that the upper end portion of the shell 3' supports only four risers 4' and that the four condensing units 2' comprise groups 5 having batches 15" of vertically extending heat-exchanging pipes 14'. Suchvertical mounting of the pipes 14' necessitates the provision of a different connection between the distributing chambers 6' and the respective risers 4". As best seen in FIG. 6, each riser 4 is connected with two radially extending supply conduits 29 each of which overlies one of the respective groups 5. The distributing chambers 6' are disposed at the upper ends of the groups 5' and are suspended on the supply conduits 29 by vertical conduits 7, 8 having abutting flanges which are bolted to each other. The portions 7' are connected to the chambers 6' and the portions 8 to the supply conduits 29. i

The collecting chambers 9' are located below the .respective groups 5 and deliver the condensate to the ringshaped conduit 12 by way of intermediate conduits 22', 10' andll' mounted in a manner as shown in FIGS. 4 and 5. The reinforcing tube 26 is connected with the risers 4' by way of conduits 25' in the same way as described in connection with FIGS. 1 to 3. All other parts shown in FIGS. 4 to 6 are denoted by reference characters'similar to those employed in FIGS. 1-3 but each followed by a prime. The compartments 75' are relatively large because the column comprises only four risers 4. It is evident, however, that the number of risers-4 or 4 (and hence the number of condensing units 2 or 2') can be varied, depending on the output of the column and the extent to which the head fraction must be cooled in the pipes 14 or adjusted in dependency on the desired temperature ofthe condensate. Such cooling action can be regulated by.

adjusting the pitch of blades or vanes 19a of the axial flow fan19. It is assumed that the condensing unit is identical with the units 2 shown in FIGS. 1 to 3, i.e., 'that the heat-exchanging pipes are horizontal or nearly horizontal and that the fan 19 forces atmospheric air against those sides of the groups which face the interior of the triangular space between such groups.

The condensate flows from the collecting chambers 9 into the conduit 11 by way of the conduits 22 and 10. The junction between the conduits and 11 contain a suitable temperature measuring device 33 which transmits signals by way of a connection 34 to the serromotor 31 of an adjusting device 32 for the blades 19a of the fan 19. The fan 19 is driven at a constant speed by an electric motor 30. The measuring device 33 may contain a thermostat which influences the serromotor 31 to adjust the blades 19a in dependency on the measured temperature of condensate in the conduits 10.

It is clear that the adjusting system of FIG. 7 can be modified in a number of ways without departing from the spirit of our invention. For example, the motor 30 can be replaced with a variable-speed motor which is controlled by the thermostat of the temperature measuring device 33 to elfect gradual or stepwise acceleration or deceleration of the blades 19a. Also, the system of FIG. 7 can utilize a variable-speed or a reversible-polarity motor 30 as well as the adjusting means 32 for the blades 19a.

It is preferred to provide a separate adjusting system for each of the several fans 19 so that the temperature of condensate can be varied in any selected unit 2 independently of the other units. The adjustments are preferably effected for the purpose of insuring that the temperature of condensate in each of the condensing units 2 at least approximates a predetermined optimum temperature. Such adjusting systems can readily compensate for any, even minimal, influence of rain, wind, sleet or snow on the temperature of the condensate.

If a particular condensing unit isexposed to strong winds, the throughput of the respective fan 19 can be reduced with attendant savings in energy. The pressure generated by the wind is then utilized to assist the fan in circulating air at the desired rate in order to insure that the temperature of the condensate remains within the optimum range.

An important advantage of the improved condenser 1 or 1' is that the groups 5 or 5 extend outwardly from the respective risers 4 or 4' to thus permit accommodation of a large number of heat-exchanging pipes in a smalLarea. The number of such pipes considerably exceed's'that in a conventional polygonal condenser of equal bulk but much smaller capacity. Moreover, the regulation of cooling action in selected units of the condenser can be carried out with a high degree of accuracy and efiiciency because the units are practically independent from each other.

The extent to which the groups 5 or 5' project outwardly beyond the outline of the shell 3 or 3' is rather small so that the condenser 1 or 1 need not be mounted on or supported by auxiliary frameworks or the like. The placing of the risers 4 or 4' in or close to the outline of the column brings about the advantage that such risers are supported by the stablest portion of the shell 3 or 3. Additional stiffening of the condenser is achieved by the provision of conduits 25, 25 and 25a which are designed to take up compressive and/or tensional stresses.

Each fan 19 or 19' can be started, adjusted and/or arrested independently of the other fans so that temporary inactivation of one or more fans need not entail full interruption of the operation. The conduits 7-8, 7'-8' and/or 29 preferably contain valves (not shown) which are closed when the corresponding fans 19 or 19 are arrested. Such closing can take place in automatic response to stoppage of the fan motors. The aforementioned valves may constitute floats or they may be replaced by siphons or other suitable fluid flow interrupting means.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can,

by applying current knowledge, readily adapt it for various applications without omitting features which fairly constitute essential characteristics of the generic and specific aspects of our contribution to the art and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

We claim:

1. In a distilling appartus or rectifying column producing a hot lower boiling fraction, a combination, comprising an upright shell wherein the hot lower boiling fraction rises, said shell having an upper end portion; a plurality of tubular risers projecting circumferentially spaced from each other upwardly from and communicating directly with said upper end portion to receive said fraction; a condenser comprising a plurality of condenser units, one for each riser, and each including two groups of heat-exchanging pipes, each of said groups being located in a substantially vertical plane and each comprising at least one bundle of interconnected pipes, the groups of each unit constitutes the sides of a triangle whose apex faces and is connected with the respective risers so that said groups project outwardly from and are supported by said risers, and the heat exchanging pipes of said groups communicating with the respective riser, and means for circulating a cooling fluid along the external surfaces of said pipes to withdraw heat therefrom and thus effect condensation of the fraction in said pipes; an upright reinforcing member surrounded by said users and supported by said upper end portion; connecting means securing said risers to said reinforcing member; and collecting means for receiving the condensate from said pipes.

2. A combination as defined in claim 1, wherein the upper end portion of said shell is of circular outline and the axes of said risers intersect a circle whose center is located on the axis of said upper end portion, said risers being equidistant from each other.

3. A combination as defined in claim 1, wherein said reinforcing member is a tube which communicates with and receives the fraction from said upper end portion, said connecting means comprising conduits for conveying the fraction from said tube into the respective risers.

4. A combination defined in claim 1, further comprising reinforcing means connecting said risers to each other.

5. A combination as defined in claim 1, wherein said fluid coolant is atmospheric air and said circulating means comprises at least one discrete air circulating device for each of said units.

6. A combination as defined in claim 1, wherein said fluid coolant is atmospheric air, said circulating means comprising at least one discrete air circulating device for each of said units, said devices being located at the bases of the respective triangles and being arranged to force atmospheric air against those sides of the respective pairs of groups which face each other.

7. A combination as defined in claim 6, further comprising a housing for each of said units, said housings having inlets for air which is circulated by the respective devices and outlets disposed between the pipes of the respective pairs of groups.

8. A combination as defined in claim 7, wherein said housings consist of sheet metal.

9. A combination as defined in claim -1, wherein said units are separated from each other by compartments and said circulating means comprises a discrete air circulating device for each of said units, said devices being arranged to circulate air across the pipes of the respective groups and into the adjoining compartments, and further comprising envelopes partially enclosing said cornpartments to prevent exchange of heat between cooler air which is forced by said devices against the pipes of the respective groups and warmer air which enters said compartments.

10. A combination as defined in claim 9, wherein each of said envelopes comprises a substantially vertical panel extending between the adjoining groups of pipes at a point remote from the risers and a substantially horizontal panel which seals the respective compartment from below.

11. A combination as defined in claim 1, further comprising a housing for each of said units, each of said housings having an outer wall and a top wall and said circulating means comprising a plurality of discrete air circulating devices, one for each of said units and each installed in one wall of the respective housing to force atmospheric air toward those sides of the respective groups which face each other whereby the air passes between the pipes of the respective groups.

12. A combination as defined in claim 1, wherein said risers are adjacent to the periphery of said shell.

13. A combination as defined in claim 1, wherein said risers have lower end portions extending in part beyond the outline of said shell.

14. A combination as defined in claim 1, wherein said' risers have lower end portions outwardly adjacent and welded to said shell, said lower end portions having inlets communicating with the interior of said shell by way of apertures provided in said upper end portion.

15. A combination as defined in claim 1, wherein said pipes are substantially horizontal.

16. A combination as defined in claim '1, wherein said pipes slope outwardly and downwardly from the respective risers.

17. A combination as defined in claim 1, wherein each of said groups comprises at least one distributing chamber which supplies said fraction to the respective pipes and substantially horizontal conduit means connecting said distributing chambers with the respective risers.

18. A combination as defined in claim 17, wherein each of said conduits comprises a first portion rigid with the respective riser and a second portion rigid with the respective distributing chamber, said portions having abutting flanges sealingly connected to each other.

19. A combination as defined in claim 1, wherein said pipes are substantially vertical.

20. A combination as defined in claim 19, wherein each of said groups comprises at least one distributing chamber communicating with the upper ends of the respective pipes, a substantially horizontal supply conduit 10 extending outwardly from and communicating with the respective riser, and at least one substantially vertical conduit connecting the supply conduit with the respective distributing chamber.

21. A combination as defined in claim 20, wherein each of said vertical conduits comprises a first portion rigid with the respective supply conduit and a second portion rigid with the respective distributing chamber, said portions having abutting flanges sealingly secured to each other.

22. A combination as defined in claim 1, wherein said circulating means comprises a plurality of discrete air circulating devices, one for each of said units and each arranged to draw air across the pipes of the respective groups.

23. A combination as defined in claim 1, wherein said circulating means comprises a plurality of discrete air circulating devices, at least one for each of said units and each arranged to blow air against the pipes of the respective groups.

2.4. A combination as defined in claim '1, wherein said circulating means comprises a plurality of discrete adjustable air circulating devices, at least one for each of said units and each arranged to convey air at a variable rate across the respective pipes, and further comprising means for measuring the temperature of condensate in each of said units and means for adjusting the respective air circulating devices in accordance with the results of such measurements.

References Cited UNITED STATES PATENTS 3,328,264 6/1967 Wartenberg 202--189 3,175,960 3/1965 Kassat 202-189 3,165,455 l/l965 Rose et a1 202l89 FOREIGN PATENTS 927,918 6/1963 Great Britain 202 661,435 4/1963 Canada 971,480 9/1964 Great Britain 165 900,407 7/1962 Great Britain 165-l22 WLLBUR L. BASC-OMB, JR., Primary Examiner D. EDWARDS, Assistant Examiner US. Cl. X.R. 

